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Proteins are polymeric macromolecules made up of subunits of amino acids. As you might expect, amino acids all contain an amino group and a carboxylic acid group; what differentiates them is a variable portion referred to as the R group. In a sense, proteins are structurally simple, since every one consists of a number of amino acids linked by peptide bonds. There are twenty different amino acids, however, which can be linked together in any order, and a typical protein contains anywhere from 30 to 1,000 amino acids. Thus, a remarkably vast diversity of different proteins is possible, and this is exactly what we find. There are probably close to 100,000 different proteins in the human body, each with a different function. The remarkable diversity of protein function is made possible by the fact that once a chain of amino acids is linked together (also called a polypeptide), it undergoes additional folding so that the final protein molecule exists in a particular three-dimensional conformation. It is this shape that allows it to function in a unique way. We can identify four levels of protein structure.



An amino acid

  1. Primary structure: A protein’s primary structure simply refers to the linear order of amino acids it contains. 
  2. Secondary structure: The secondary structure of a protein comes about due to local interactions, usually hydrogen bonds between atoms of adjacent amino and acid groups. Common secondary structures include the alpha-helix and the beta-pleated sheet. 
  3. Tertiary structure: A protein’s tertiary structure refers to its ultimate three dimensional shape. It folds uniquely due to long range interactions between the R groups of the amino acids. Such interactions include hydrogen bonding, electrostatic interactions, and hydrophobic interactions. It is the tertiary structure that is responsible for the protein’s function. 
  4. Quaternary structure: Not all proteins have a quaternary structure; only those that consist of multiple polypeptide chains. Quaternary folding refers to the interactions between multiple chains of amino acids to achieve a protein that can only function in this complex state. 
Proteins perform a vast array of functions, acting as enzymes, antibodies, structural components, hormones, and a wide variety of other functional entities. Well-known proteins include:
  • Hemoglobin, which helps carry oxygen in the blood;
  • Collagen and Keratin, major components of skin, hair, and connective tissues;
  • Insulin, a hormone that regulates blood glucose levels;
  • Pepsin, an enzyme that digests other proteins in the stomach and others too numerous to list!


Hierarchical folding of a hypothetical protein, aa: amino acid.

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